A problem frequently arises in the case of induction hobs or cooktops, in which audible noises can arise when operating several hotplates. In part, these noises are considered to be unpleasant to an operator, not only as a result of the noise per se, but also because it may imply to the operator that an induction hob is malfunctioning. The sensation of noise is also dependent on the sound level intensity and the coincidence with the human audible frequency range, i.e., as a function of the frequency of the noise.
There are various causes of such noise. First, magnetic field control ferrites are provided underneath the induction coils, which are subject to magnetostriction, i.e., a change in length as a function of the induction coil operating frequency. This, in part, may also apply to the cooking utensils used. Although the operating frequency of induction coils is normally above the audible range, the noise can be audible as a result of intermodulation with another operateing induction coil. Audible mixture sound can arise from the frequency difference of the operating frequencies and their harmonic waves. Further, intermodulations can occur if two frequency converters for the induction coils are connected to a common supply voltage. In this case, the supply voltage for a second frequency converter is modulated by the first frequency converter.